Field of the Invention (Technical Field)
The present invention relates to an electrolytic on-site generator which is nearly free of maintenance.
Background Art
Note that the following discussion refers to a number of publications and references. Discussion of such publications herein is given for more complete background of the scientific principles and is not to be construed as an admission that such publications are prior art for patentability determination purposes.
Electrolytic technologies utilizing dimensionally stable anodes have been developed to produce mixed-oxidants and sodium hypochlorite solutions from a sodium chloride brine solution. Dimensionally stable anodes are described in U.S. Pat. No. 3,234,110 to Beer, entitled “Electrode and Method of Making Same,” wherein a noble metal coating is applied over a titanium substrate. Electrolytic cells have had wide use for the production of chlorine and mixed oxidants for the disinfection of water. Some of the simplest electrolytic cells are described in U.S. Pat. No. 4,761,208, entitled “Electrolytic Method and Cell for Sterilizing Water”, and U.S. Pat. No. 5,316,740, entitled “Electrolytic Cell for Generating Sterilizing Solutions Having Increased Ozone Content.”
Electrolytic cells come in two varieties. The first category comprises divided cells that utilize membranes to maintain complete separation of the anode and cathode products in the cells. The second category comprises undivided cells that do not utilize membranes, but that also do not suffer nearly as much from issues associated with membrane fouling. However, it is well accepted that one of the major failure mechanisms of undivided electrolytic cells is the buildup of unwanted films on the surfaces of the electrodes. The source of these contaminants is typically either from the feed water to the on-site generation process or contaminants in the salt that is used to produce the brine solution feeding the system. Typically these unwanted films consist of manganese, calcium carbonate, or other unwanted substances. If buildup of these films is not controlled or they are not removed on a fairly regular basis, the electrolytic cells will lose operating efficiency and will eventually catastrophically fail (due to localized high current density, electrical arcing or some other event). Typically, manufacturers protect against this type of buildup by incorporating a water softener on the feed water to the system to prevent these contaminants from ever entering the electrolytic cell. However, these contaminants will enter the process over time from contaminants in the salt used to make the brine. High quality salt is typically specified to minimize the incidence of cell cleaning operations. Processes are well known in the art for purifying salt to specification levels that will avoid contaminants from entering the cell. However, these salt cleaning processes, although mandatory for effective operation of divided cells, are considered too complicated for smaller on-site generation processes that utilize undivided cells.
U.S. patent application Ser. No. 11/287,531, which is incorporated herein by reference, is directed to a carbonate detector and describes one possible means of monitoring an electrolytic cell for internal film buildup. Other possible means for monitoring carbonate buildup in cells that utilize constant current control schemes is by monitoring the rate of brine flow to the cell. As brine flow increases, it is usually, but not always, indicative of carbonate formation on the cathode electrode which creates electrical resistance in the cell. Other than these methods and/or visual inspection of the internal workings of a cell, there currently is not an adequate method of monitoring the internal status of the buildup on an electrolytic cell.
The current accepted method of cleaning an electrolytic cell is to flush it with an acid (often muriatic or hydrochloric acid) to remove any deposits which have formed. Typically, manufacturers recommend performing this action on a regular basis, at least yearly, but sometimes as often as on a monthly basis. Thus there is a need for a more reliable method for insuring cleanliness of the electrolytic cell is to perform a cleaning process on an automated basis that does not require the use of a separate supply of consumables such as muriatic or hydrochloric acid, and that does not require operator intervention.
U.S. Pat. No. 5,853,562 to Eki, et al. entitled “Method and Apparatus for Electrolyzing Water” describes a process for reversing polarity on the electrodes in a membraneless electrolytic cell for the purpose of removing carbonate scale and extending the life of the electrolytic cell. This method of electrolytic cell cleaning is routinely used in flow through electrolytic chlorinators that convert sodium chloride salt in swimming pool water to chlorine via electrolysis. However, currently used flow through electrolytic cells are constructed of electrodes (anode and cathode) that both have common catalytic coatings. As electrical polarity is changed, the old cathode becomes the anode, and the anode becomes the cathode. Special catalytic coatings have been developed for these applications, For instance, Eltech Corporation has developed the EC-600 coating specifically for the swimming pool chlorination market. Sodium chloride is typically added to the pool water raising the total dissolved solids (TDS) content to approximately 4 to 5 grams per liter. At these TDS values, the current density in the swimming pool electrolytic cells is relatively low. The special anode coatings for pool applications are designed to tolerate these low current densities for extended periods with polarity applied in either direction. However, most dimensionally stable anodes for chlorine production in membraneless electrolytic cells producing chlorine at 8 gram per liter (8,000 mg/L) concentration of free available chlorine (FAC) cannot tolerate high current densities (greater than approximately 1 amp per square inch) in reverse polarity mode. Thus, although simply reversing the polarity works for low current density electrolytic cells, it will not work for electrolytic cells which normally operate at a high current density, since the anode will be damaged if high current density is applied during the reverse polarity cleaning operation.
One of the other maintenance items for electrolytic generators is the requirement that operators occasionally measure and set water flow into the system. The flow through the generator can vary greatly with incoming and outgoing water pressure and/or contaminant buildup in the system or electrolytic cells. Typically, measurements are made with either flowmeters or with timed volume measurements, and adjustments to the flow are performed with manual valves. Keeping the electrolytic generator operating within flow specifications is important, as it ensures reliable long term operation the generator within its efficiency specifications.